In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed. Various metal oxides are used for a variety of applications. Of them, indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Note that a thin film transistor is an element having at least three terminals of a gate, a drain, and a source. The thin film transistor has a channel region between a drain region and a source region, and current can flow through the drain region, the channel region, and the source region. In this specification, the term “source” and the term “drain” are not distinguished. When one of terminals functioning as a source or drain is referred to as “source”, the other is referred to as “drain”.
Some metal oxides have semiconductor characteristics. Metal oxides having semiconductor characteristics include, for example, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor including an oxide semiconductor has high field-effect mobility. Thus, with use of the thin film transistor, a driver circuit for a display device or the like can be formed. A thin film transistor in which a channel formation region is formed using such a metal oxide having semiconductor characteristics is already known (Patent Documents 1 and 2).